Liquids, such as aqueous liquids, are purified to remove unwanted ions by passing the liquid through a packed column of ion exchange resin beads. In order to improve efficiency of ion removal and rate of processing the liquid, small particle size ion exchange resin beads and high flow rates are desirable. While smaller particle size resin beads improve efficiency in a packed column, they also effect a decrease in the fluid flow rate which, in turn, renders the optimization of the purification process utilizing the beads difficult. A common undesirable phenomenon when using a packed column of beads is the phenomenon of channeling wherein the liquid being purified passes only through a portion of the bed while rendering the remainder of the bed underutilized.
A significant problem associated with incorporating ion exchange resin particles into a polymer matrix is that the resin particles are swellable in aqueous liquids. Thus, when a composite material comprising a polymer binder and the ion exchange resin particles is contacted with water, in the case of a porous membrane composite, the porosity of the composite is significantly reduced thereby significantly reducing the flow rate of the liquid through the porous composite.
Ultrahigh molecular weight polyethylene is a desirable material since it exhibits good chemical resistance to a wide variety of reagents and therefore provides wide flexibility as a material for uses in processes involving contact with these reagents such as in purification processes. Hydrofluoric acid, for example is a reagent commonly employed in the treatment of silicon-based materials such as silicon-wafers used as a base to form integrated circuits. A currently available process for purifying hydrofluoric acid is disclosed for example in U.S. Pat. No. 4,952,386. This process is less desirable since it relies upon the use of columns of ion exchange resin beads.
Accordingly, it would be desirable to provide membranes having ion removal capacity which have high ion capture (efficiency) characteristics, have a high ion removal capacity per unit area and permit maintenance of desirable flow rate per unit area through the membrane when it is wet in aqueous solution. In addition, it would be desirable to provide such membranes which are useful in processing corrosive reagents, including low pH acids such as hydrofluoric acid.
This invention pertains to a method for removing selected ions (e.g., metallic ions) and particulate material from an aqueous acid solution using particle removing membranes (e.g., ultra high molecular weight polyethylene) having immobilized ligand groups that possess capacity and high equilibrium binding constants for ion removal, even in acid. The method is particularly useful for simultaneously filtering/purifying aqueous acids, such as hydrofluoric or hydrochloric acid.
According to the method of the invention, metallic ions and particulate material are simultaneously removed from an aqueous acid solution by contacting an aqueous acid solution which is contaminated with metallic ions and particulate material with a composition suitable for removing metallic ions and particulate material contained in said solution, then recovering a purified and filtered solution essentially depleted of metallic ions and particulate material. Compositions useful for purifying and filtering comprise an ion-binding ligand bound to a membrane, having an affinity for metallic ions and having an ability to remove particulate material contained in said solution. The membrane ligand combination is represented by the formula:
Mxe2x80x94Bxe2x80x94L 
wherein M is a membrane or composite membrane derivatized to have a hydrophilic surface and containing polar functional groups; L is a ligand (e.g., a macrocycle or other similar chelating ligand) having an affinity for metallic ions and containing a functional group reactive with an activated polar group from the membrane; and B is the covalent linkage formed by the reaction between the activated polar group and the functional group of the ligand. In a preferred embodiment, M is capable of removing particulate material contained in the solution. In another embodiment, the membrane will comprise a plurality of different ligands that are ion specific.
The filtration/purification methods of this invention have several advantages. The fluid to be processed can flow through a membrane structure and react with the ligand that is immobilized on the membrane inner surface with very small mass transfer resistance. This allows the fluid to be processed through membranes at relatively high throughputs with no loss in ligand-ion complexing efficiency. The particle retention properties of membranes have been combined with ligand technology in one system to remove both ions and particles from aqueous acid solutions.
The invention further pertains to filtration/purification devices comprising membranes or composite membranes with immobilized ligand groups. The ligand immobilized membranes have been fabricated into devices that enable high flow rates and low pressure drops. These engineering requirements may not as easily be met with bead technology.